1. Field of the Invention
This invention relates to a three-phase, half-bridge power inverter, and more particularly, to a unique control waveform for switching the poles of an inverter to balance harmonic cancellation between waveform notching and low impedance filtering.
2. Description of the Prior Art
Power inverters are known generally and are devices which serve to transform DC (direct-current) electrical energy, such as from a fuel cell or the like, into AC (alternating current) electrical energy which is the primary form used by power utilities. Such devices most often include one pair of switches, called a pole, connected as a half-bridge or full bridge in series across the positive and negative busses from the DC energy source. By actuating the switches in each pole, a series of positive and negative going pulses are formed with have a primary component corresponding to the fundamental desired AC sinusoidal waveform. Generation of AC electrical energy by such means inherently results in the generation of the harmonic components in addition to the fundamental AC sinusoid which must be removed or suppressed by some means before the load current can be supplied to an AC device or an electric power untility. Although numerous techniques for reducing the harmonic content in an inverter output have been proposed, a combination of pulsed-width modulation together with some in-line filtering has resulted in a relatively high-quality sinusoidal output signal.
With the advent of solid state technology to the inverter field, power switching devices such as power transistors, thyristors, etc., and integrated circuits have allowed a significant reduction in inverter components, size and weight. The filters employed in an inverter circuit tend to be one of the more costly and weighty items resulting in the development of numerous techniques which minimize the amount of filtering required to synthesize an acceptable AC waveform. In addition, the use of high impedance filters to remove harmonics requires a large series coupled inductor and voltage regulation is normally necessary to compensate for the voltage drop across this inductor as the loading on the inverter varies.
In U.S. Pat. No. 3,614,590 issed Oct. 19, 1971 to A. Kernick a solid state inverter is described utilizing a digitally derived control waveform for the full bridge power switching devices. The controlling waveform utilizes voltage pulses of uniform width interdispersed by spaces which may be of nonuniform width all produced relative to the integral number of equal increments. As indicated in FIG. 4 of this reference, a number of programmed waveforms are disclosed for the switching devices which serve to reduce the harmonics in the inverter output.